Orthopaedic bone cement is used throughout the world to secure hip, knee and other metallic prostheses in an appropriate medical position. Also, bone cement can be used to replace and/or repair damaged bone, such as a bone void filling and spinal column with degenerative intervertebral discs by permanently stabilising adjacent vertabrae by fusion.
Bone cement for both joint surgery and bone void filling is generally provided as two or more components, often a powder and a liquid, that, when thoroughly mixed together, form a paste or cement. Thorough mixing of the components is necessary to avoid brittle or “hot” spots. After mixing, the cement generally has to be used quickly before it sets hard.
Bone void filling is used in the repair of osteoporotic bone, typically after fracture, by stabilising the bone and allowing bone growth. In some cases bone growth stimulants are added to the materials to hasten healing. Calcium based sulphates and phosphates materials are typically used. The principles of mixing apply equally to both PMMA bone cement and bone void filling cements.
Bone cement is produced by thoroughly mixing together two components, usually methylmethacrylate monomer liquid and polymethylmethacrylate (PMMA) powder. This type of material is typically used in joint surgery including hips, knees and small joints such as the shoulder, hand and wrist, and foot and ankle.
The mixing is usually carried out using a simple bowl and spatula. The liquid and powder components are put into the bowl and the surgeon or assistant uses a pestle or spatula to thoroughly mix the components. The surgeon then removes the required amount of cement and manipulates it by hand before inserting it into a preformed cavity or applying it to a resected bony surface where the prosthesis is to be positioned. Cement may be applied by hand or may be put into a syringe and applied separately. However, this simple mixing has two major drawbacks.
Firstly, free methylmethacrylate fumes are emitted from the mixture. It is desirable to remove these fumes, or prevent them from escaping into the atmosphere, since they have an unpleasant odour and may be harmful to operating room personnel. The fumes are known to cause nausea and giddiness and are generally objectionable, particularly to the nurses who carry out the mixing. There has also been concern that long term exposure to these fumes results in a more serious health risk. Current employment law relating to occupational health mean medical staff must now be protected against the exposure to hazardous substances.
Secondly, a very high mixing efficiency is required to produce a homogenous cement material. During the mixing process, air is naturally introduced into the mixture since air is inherently existent within the powder and also in and around the mixing vessel. Air bubbles are also produced by the “boiling off” of monomer which occurs during the mixing process. The introduction of air produces a weak cement and, since the joint must usually support a heavy load, it is important to reduce the amount of air in the mixture as much as possible in order to improve the mechanical strength of the cement material.
In order to eliminate as much air as possible from the mixture, desirable for most types of cement, mixing is now preferably carried out in a closed vessel, most preferably under vacuum. This considerably reduces the amount of air in the mixture. Mixing in a conventional bowl and spatula system can produce a product with a porosity value of 20% to 25%. In a vacuum mix, the porosity value is often reduced to levels below 5%.
As mentioned above, calcium phosphates and sulphates can be used as another type of bone cement material, which does not necessarily need to be mixed in a vacuum. Conventionally, this type of bone growth stimulant is prepared in paste form and is delivered to an application site.
Several devices for mixing the cement are available. Some of these are in the form of hand-held mixing bowls as mentioned above. WO 93/10892 describes an improved bowl mixer. The substances to be mixed are placed by means of a rotating paddle extending into the bowl to which a vacuum is applied. The substances are mixed by means of a rotating paddle extending into the bowl which is rotated manually by means of a handle extending through the lid of the bowl. In some applications, an example of which is disclosed in WO 93/10892, bowl mixing is favoured. Many surgeons prefer to “hand pack” the cement. Bowl mixing tends to be preferred by nurses who are used to the convenience of mixing in this vessel; a bowl is easier to use and it is important that nurses feel confident since timing is very crucial and the mixture must be “right first time”. Many surgeons also tend to prefer bowl mixers because they can easily take samples of the cement from the bowl at any time to determine the progress of polymerization as it is crucial that the mixture does not begin to set before it is applied.
However, in some applications, it is preferable or necessary to apply the mixed cement to the bone by means of a syringe. Indeed some surgeons, particularly in Europe, prefer syringe-type application to “hand packing”. If the cement is mixed in a bowl, it must then be transferred to a dispensing syringe which can be messy and time consuming and may expose the mixture to more air entrapment. This problem has been overcome by combining a mixing chamber with a syringe. Although advantages can be obtained with a simple closed mixing chamber/syringe combination, creating a vacuum can provide additional advantages. For example, EP 0178658 discloses a device for mixing bone cement comprising a mixing container connected to a feed device. A vacuum source may be connected to the feed device for mixing the substances under vacuum. This device has proved to be a very efficient mixing and transfer system and eliminates the need to transfer the mixed cement from the mixing bowl to a syringe.
However, the mixing paddle of EP 0178658 is rotated by a rotary electric drive motor. This makes the device costly and space consuming and requires specialist and time-consuming installation. The device is not easily portable and its use is, therefore, not particularly flexible.
U.S. Pat. No. 4,758,096 and U.S. Pat. No. 3,606,094 also disclose bone cement mixers in which the cement is mixed in the dispensing vessel. In U.S. Pat. No. 4,758,096, the mixing is effected manually by means of a “masher” plate-type agitator. The masher plate is attached to a shaft attached to a handle. The agitator is movable in the chamber both axially and rotatably to permit mixing of the cement by the user moving the handle vertically and rotatably.
In the device of U.S. Pat. No. 3,606,094, the mixing element comprises an elongate conduit having paddle projections. The paddles are rotated by a rotating motor or by hand.
EP 0744991 discloses a bone cement mixer in which the mixing chamber forms the body of a dispensing syringe wherein a nozzle can be attached to one end of the mixing chamber so as to dispense the bone cement.
GB 2411849 discloses an apparatus for mixing bone cement and discharging the mixed bone cement from a mixing container into a discharging device such as a syringe or syringes.
Another product on the market that provides a mixing and dispensing device is the HiVac™ 7 (provided by Summit Medical Ltd., see http://www.summit-medical.co.uk/product/hivac7.html). This device allows for mixing, e.g. biologics, in a mixing chamber. The HiVac™ 7 provides a mixing rod having a diameter of 8 mm. Once the mixing phase is complete, a cap can be lifted from one end to reveal a “luer” connection to which a nozzle can be attached. The mixing chamber then also acts as a dispensing chamber and the bone cement can be dispensed through the nozzle.
The bone cement mixing and dispensing devices discussed above have the advantage of providing an apparatus that both mixes and dispenses bone cement material.
Generally, these mixing apparatuses discussed are made to hold a standard volume of cement of 40 cc or more in the mixing and dispensing chambers. Sometimes, however, there is a need for only a small amount (e.g. 10 cc) of cement, for example for smaller joints such as ankles etc., where a volume of 0.5-20 cc, for example, is needed; or for a combination of calcium based sulphates and phosphates. The bulky nature of chambers having, say, 40 cc volume is not easily maneuvered in a smaller target site (for example, ankle joints etc.). Also, the cement components are expensive and use of a greater volume than needed is wasteful and costly. Simply reducing the size of the apparatuses discussed above causes the components therein to become weak and to easily break. For example, simply reducing the size of the components shown in the HiVac™ 7, and particularly the rod, makes these components weak and prone to breaking. There is therefore a need for a device that can cope and assist in treatment at smaller target sites.
Further, optimum mixing occurs in the above devices when the chambers are mostly full—e.g. ˜80% full. When a chamber is ˜80% full, this allows for a mixer paddle or disc to push through the material and have the resistance from the bone cement material, caused by the material being compressed against the chamber bottom to push the material through the paddle or disc. As the paddle or disc is then returned through the chamber, the material is again pushed through the paddle or disc by resistance. However, if the devices discussed above were to be used for treatment in smaller target sites, it would be necessary to use a smaller amount of bone cement material that would be less than the volume of the chambers discussed above. Simply providing a smaller amount of bone cement material into a chamber with, say, 40 cc volume, or more, has disadvantages. For example, if the chamber were only ˜40% full, the material simply clings to the paddle or disc until the paddle or disc hits the bottom of the chamber where it would be pushed through the paddle. Before mixing the material again, the paddle or disc has to travel ˜60% of the chamber, which does not provide efficient and optimum mixing. Therefore, there is a need for a device that provides adjustable volumes in the chamber to allow for a range of volume of bone cement material.
The present invention aims to overcome the above-mentioned problems. Of course, several types of bone cement material have been discussed above, but the present invention is not so limited to these types of bone cement material. The present invention aims to provide a device for known types of bone cement material and bone cement material that may be developed in the future.